Take out your homework
The pH of a 0.150 M solution of a weak acid is
4.10. Calculate the Ka value of the acid.
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Saturday School: 10-11ish
Monday: Cold Call Quiz
How do we know when to use an ICE table to
determine pH? Consider the following two
problems
1.
2.
Determine the pH of a solution of 0.050 M
HCl
Determine the pH of a solution of 0.080 M
HF
Give each group a different problem
Consider a solution of Weak Acid
HA D H+ + A1. Determine the pH of the solution if [HA] = ? M
2. What species is present in the greatest
concentration?
3. On your whiteboard, model the relative
concentrations of each species
HNO2 Ka = 4.0 x 10-4
HCOOH Ka = 1.77 x 10-4
HF Ka = 7.2 x 10-4
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What can we conclude about the relative
concentrations of each species of a weak acid at
equilibrium?
• What is present in greatest quantity?
• What is present in the lowest quantity?
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Consider the following two solutions
• HNO2: pH = 3.5
• HCl: pH = 3.5
Which solution has the greater concentration of
hydronium ion in solution?
•
Kw = Ka * Kb
•
Calculate the Ka for the ammonium ion (Kb = 1.8 x
10-5)
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Consider the following two acids and their
ionization constants:
HCOOH
Ka = 1.7 x 10-4
HCN
Ka = 4.9 x 10-10
Which conjugate base (HCOO- or CN-) is stronger?
Explain.
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Polyprotic acids: Acid that can donate more than
one H+
Calculations of Ka are more complicated because
you need to take into account all potential
ionizations
We’ll focus just on monoprotic acids
Acid Strength based on structure
Increasing size and EN leads to an increase in acid
strength
Essentially, if molecule can ionize easier, it is a
stronger acid
Increasing size and EN weakens bond